This invention relates to waxes, a process for producing same as well as to compositions containing such waxes.
It is conventional to utilize waxes, for example, in printing ink and varnish systems in amounts of between 0.5 and 5% by weight, based on the total weight of the system, in order to attain or improve certain properties in printing inks and varnishes.
Micronized waxes (i.e., waxes having a particle size in the micron (um) range) exhibit an advantage over non-micronized products in that they can be added either directly or in the form of a dispersion prepared in the cold state.
Two basic methods exist for the production of micronized waxes.
The first method is a spraying process wherein a hot, low-viscosity wax melt is forced through a nozzle and atomized into very fine wax droplets. During the subsequent cooling step, these wax droplets solidify to form spherical particles. If these particles do not as yet exhibit a particle fineness sufficient for the respective usage, a screening procedure follows which removes the coarse particles still contained therein.
The second method is a mere grinding process. In this procedure, waxes present in the form of pastilles, prills, scales or coarse wax powders, are micronized by means of special grinding installations, for example jet mills. Some of these mills are constructed so that only ground material having the desired particle size will leave the grinding chamber. Such mills have, so to speak, a screening unit integrated into the milling chamber.
Furthermore, series-arranged grinding units are known which bring the waxes to the desired grain size in various stages. Finally, milling unit-screening unit combinations are utilized as well wherein the coarse product screened out after the first grinding pass is immediately returned to the grinding chamber and brought to the desired particle fineness in a second grinding pass.
In the first mentioned spraying process, substantially spherical, regular particles having a small surface area are obtained. By substantially spherical is meant a more or less accomplished ball-form, whereby a small part of the particles may possess an ellipsoidic or short bar form, always having rounded contours. On the other hand, the grinding processes yield broken, irregular particles having a large surface area.
When added to printing ink or varnish compositions, spherical particles improve the non-sticking characteristics ("slip"), as well as abrasion resistance and scratch resistance, of printing ink films and varnish films formed by applying the compositions to substrates. With regard to these properties, spherical particles yield far better results than broken particles of the same wax body composition. Another advantage provided by the spherical particles is that due to their spherical and smooth surface, the particles impart a smooth surface to printing ink films and varnish films so that impinging light is reflected in such a way that almost no less of luster occurs due to the presence of the wax additive. In contrast thereto, ground products, i.e., broken particles, lead to disruptions in the surface of printing ink films and varnish films. Impinging light is reflected in a diffuse fashion resulting in a loss of luster that is high as compared with films containing spherical particles. Thus, broken particles produce a dulling effect in printing ink and varnish films.
A disadvantage associated with spherical particles is their poor wetting and dispersing properties in comparison to broken particles. The spherical particles are more difficult to wet than broken particles due to the smaller surface area of the former. On account of their spherical shape, they are also subjected to lower shear stress when being stirred into printing ink and varnish systems than broken particles, so that they are distributed in such systems only with great difficulty.
Micronized waxes, based on the same wax composition and the same particle size, when prepared by spraying tend to form agglomerates in printing ink and varnish systems to a far greater extent than micronized waxes when produced by grinding. For this reason, the addition of micronized wax powders produced by spraying to printing ink or varnish systems can frequently be accomplished only in a predispersed form whereas the micronized waxes obtained by grinding can, in most cases, be stirred directly into printing ink and varnish systems.